Space Time is curved?

[Eardrum73]

Hi all,

I was reading some material last week on the general theory of relativity and apparently the theory states that space time is curved!!!

According to the theory, mass curves the space time fabric....

So you could wormhole from one area to another shaving many light years of your time.

But how does mass curve space time??? :confused: is there an article somewhere in the net that can explain the finer points of mass curving spacetime in laymans terms yet without being too superficial?
or perhaps one a member could explain this clearer.... :prey:

Also, for those well verse in astrophysics... is this the general relativity the generally accepted theory amongst astronomy buffs?
or has there already been another more advance theory that is widely accepted instead of this one?

He publish this in 1910 I think.... almost 100 years ago now.

[janoskiss]

Yes nearly 100 years young and the theory is as rock solid as ever. (Newton's theory of gravitation remained unchallenged for over 300 years.) It is consistent with all observation. The latest test of one of the theory's predictions is being carried out by the gravity probe B which will measure gravitational 'drag' due to an object moving through the curved spacetime generated by another object (the Earth in this case).

The trouble with general relativity is that it does not play friendly with quantum mechanics, the other triumph of 20th century physics. What this means is that we do not understand how gravitation works on very small scales and, more importantly, we do not understand the fundamental nature of physical reality as a whole, in a consistent manner.

Check out the wikipedia pages (and links therein) on gravity and general relativity:

http://en.wikipedia.org/wiki/Gravity
http://en.wikipedia.org/wiki/General_relativity

[Jonny S]

Hello Eardrum!

This is my first post - I have been snooping around here, then joined but never posted - but that General Relativty really spiked my interest!

General Relativity is actually one of those topics that one doesn't start until the end of a Physics degree, mortals have to attempt to understand special relativity first. A great book that will give you a good understanding without "dumbing down" the material to much is "The Elegant Universe" by Brian Greene. Get this book, it will get you up to speed about current theory.

You asked, what theory is prevalent amongst scientists. Currently if you study the very big (astrophysics) Relativity describes things so extremely well that astronomers will tend to ignore the science of the very small (quantum physics). Only recently is there a push to combine these theories into something called "String Theory" and even more weird is a thing called "M- theory". Still, Relativity, together with Quantum physics,reign supreme...for now.

Now, space curvature. Think of empty space as a huge table cloth. Completely smooth, no bumps, nice smooth table cloth. The table cloth is held at the ends, aloft. Now...put a heavy object in the middle, what happens? Well the heavy object (lets say a bowling ball) bends the table cloth doesn't it. So this is an ok analogy for the Sun, it bends the space around it. Now what is so interesting about space bending is that objects travelling on this bent space will travel akwardly, in fact the travelling object will travel not in a straight line but because its "path" is bent. Warped Space is another way to say...gravity.

Hold on minute, why does mass curve space time..hmm... I can't remember what I read..but I can get back to you. Wow, I don't think I helped:rofl:sorry!

Cheers

[Eardrum73]

hihi Janokiss and Jonny S,
And congradulations on your first post Jonny S. The table cloth analogy kinda helps in the understanding. :) I am definitely one of the mere mortals with no physics degree.
All this came about with me buying my first telescope 2 weeks ago and just made me wonder about space travel and such. Before my telescope I woundn't even have found this topic remotely interesting. :whistle:
But now its like gee - I can't get enough.

[Eardrum73]

Since we are on the topic of special relativity... and since you guys sounds like Astrophysicist(I hoped I spelt that right!).....

I did "attempt" to understand Special Relativity, and my understanding so far is that the faster you travel, (rather the closer to the speed of light you get..) three things will occur, 1) Time Dilation, 2) Length Contraction, 3) Mass increases.... I am not 100% sure about the last one. But these changes are only apparent from the frame of reference of the stationary observer. if you were the traveller you would not noticed the change. Until its too late ... gasp!

According to some article I read... General Relativity is just Special relativity taking in consideration of gravity. Is this true?
But at this stage I fail to see how SR is in anyway related to bending space time.... ??!?

On another note - special relativity in terms of Black Holes, my understanding is that if you reach the singularity of the black hole you are travelling at the speed of light is this correct?
as even light cannot escape from the singularity of the black hole. Hence is it safe to assume to reach singuarity you would have to be travelling at the speed of light?
and from the the observers point of view, you would be experience the 3 SR things above?
So you would never really approach it from an observers frame of reference, (you'd get very close, but never to really reach it) since time would come close to almost a standstill (not quite but almost) in itself. Is this true?

[janoskiss]

It would want to be much better than the mini series by the same title and narrated by Greene. Very uninsightful.

The Universe and Dr. Einstein by Lincoln Barnett is an oldie but a goodie. Easy to read and understand by non-specialists. I first read it while still in high school, and was my first introduction to relativity, and it absolutely blew me away. I had friends and family with zero physics knowledge read and enjoy it too. Highly recommended.

[trav]

Have a read of Steven Hawking's "Brief History of Time" and also anything you can get your hands on by Richard P Fenyman. These two are some of the best writers on the topic there have ever been. They aim to teach the average punter about physics.

O, and something interesting I read the other day, regarding special relativity; what Einstein is quoted as saying as his biggest ever mistake (the inclusion of the constant in the theory) might now be explained by the presense of dark matter and dark energy. He might well have been right the first time.

[cahullian]

A simple way to understand the mass part. If a car hit you traveling at 3 kph it would hurt but not kill you. If it hit you at 100 kph it would kill you because the greater speed increases the mass. To travel at the speed of light would make the mass of the moving object to great for any amount of energy to propel it fast enough. I hope this helps.

If light can't escape the gravity of a black hole. Could we maybe use gravity in the future to travel????

Gazz

[ving]

mass increases with speed? thats just crazy talk.... the car kills you at 100ks because its hitting you with a greater force not because its eaten a bigmac in it travel towards you and gained weight.... sure the force can be measured in psi or kg/square cm... gah! gravity is not my cup of tea...
anyhow it is my understnding as has been mentioned, that it's gravity that bends space...

[[1ponders]]

Hi Eardrum.

A couple of things here that might help. Jonny S' description can be taken a step further. Do google search for "Einstein Rings". These are arcs of light that appear around galaxies (and sometimes stars). They are the effects of mass warping space time and turning the space around the galaxy into a sort of lense. What we are seeing in the arcs is a galaxy that is directly behind the foreground galaxy. Now we shouldn't be able to see the rear galaxy but we do in the form of these arcs. The intervening galaxy has acted like a lens and this lens is formed of space/time being warped into a shape like a lens by gravity which is a caracteristic of mass.

As for you ideas on the black hole. If an observer were to watch someone fall towards a black hole they would appear to get slower and slower and slower and never quite seem to get to the event horizon. But to the person falling towards the black hole they would experience no slowing down at all. They would continue to be drawn in by gravity at an increasing rate. Just like us jumping off a building only there would be no terminal velocity until you hit the speed of light and the singularity. After that.....Who knows. Theoretically if the black hole is rotating rapidly or has an electrical charge (don't ask me how that would be possible I'm just relaying what I read :P ) then theoretically you could possibly get to another universe/space-time/place/event??? Unfortunately you would have to survive the extreme tidal forces of the black hole first>

[ving]

they used this bending of space on the cassini probe didnt they as it was slingshot around the sun....

[[1ponders]]

Yes using the gravity well of the planet to increase the momentum of the space craft. Don't know if they did it on Cassini but they will be doing it on the new pluto mission by slinging around Jupiter. Interesting thing though is that because of the conservation of momentum the increased momentum of the spacecraft cost Jupiter momentum. Not that you'd notice though :P The moon is slowly slowing down the earth by the same process so by the time the moon breaks free of earths gravitational pull the earth will have slowed down so much that one day then will equal about 45 days now :scared:

A little bit off topic, sorry.

[Mikezoom]

Link from a friend...

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/relcon.html#relcon

[davidpretorius]

hey eardrum, table cloth is great to understand, and we commonly portray a black hole as a hole in the middle of that table cloth like a drain pipe

but remember we are talking only in two dimensions. a black hole is three dimensional ie no matter which side of this "black hole ball" you are looking at there is no pipe going anywhere. matter goes in and does not come out. it will head towards the centre of the sphere.

So while we think of a worm hole as two pipes joining two 2d table cloths together, this actually has to happen as 2 x 3d pieces of space somehow joined. the brain starts to fall down here!!!

[cahullian]

Sorry about the confusion Ving I meant to say " relativistic mass" but I was in a bit of a hurry.:whistle:

[janoskiss]

There is no difference between mass and "relativistic mass". Mass is mass. It is a relativistic effect that the mass increases with speed. But this increase is not appreciable until the object is travelling at an appreciable fraction of the speed of light. At 100 kph a car has the same mass as at rest, for all practical purposes. The car would need to be travelling at 10 million kph for the mass to increase only slightly. Beyond that the mass rises more and more rapidly with speed, and approaches infinity at the speed of light, which is another way of saying you cannot ever get to the speed of light.

The car doing more damage at 100 kph than at 3 kph is due to increased kinetic energy (energy due to motion) and it is a classical effect, well understood since the time of Newton. That is not to say there are two theories, i.e. one for low speeds and one at high speeds. Relativity reduces to classical Newtonian mechanics at ordinary speeds.

[Nightshift]

Just to add a twist to the conversation, if light (in a vacuum) travels at the speed of light, (a fair assumption) and a black holes gravitational pull is greater than the velocity of light (making it "black"), and the general theory of relativity suggests nothing can travel faster than the speed of light, then..... The light trapped by a black hole must in fact be stationary, not moving towards the centre as suggested, to move backwards towards the centre it would have to be dragged back faster than the speed of light. Problem: Balck hole theory is wrong or Einstein was wrong....what gives???

[janoskiss]

"Pull" (i.e., force) and speed are different concepts, so you cannot say one is greater than the other. Gravity bends the path of light, not speed it up or slow it down. Another way to put it is that light follows the curvature of spacetime.

[AGarvin]

It's actually Special Relativity that sets the "nothing can travel faster than light" rule. General Relativity describes the geometry of space. If you sit just on the event horizon of a blank hole, light for you would still travel at the speed of light as Special Relativity says that light travels at the same speed for all observers. From a General Relativistic point of view however, an observer way outside the event horizon would see you slowed down to the point of not moving. This is because, as Janoskiss said, that it's the extreme curvature of spacetime that your in that's causing the dilation.

Another simpler way to think of it is that the escape velocity on the event horizon equals the speed of light. Beyond (inside) the horizon, it's greater than c.